ORIGINAL: da Rock
You can easily do that by finger choking. Put your thumb over the opening in the carb to plug it tightly and turn the prop a revolution by hand. Watch the delivery tubing while you do it. It should only take a couple of turns until fuel shows up being pulled to the carb. Used engines sometimes have crud in the tank, fuel tubing, or inside the needle valve. Back pumping sometimes pushes the blockage back into the tank. Try to fill the tank from the engine's fuel line. It will at least show you how clear the path is from the carb back. While the line if off the carb, try to pump right into the carb. If it seems blocked, pull the needle completely out and pump again.